MBE process for preparing oxide superconducting films

ABSTRACT

In a process for preparing a thin film of oxide superconductor having a layered crystal structure by depositing each layer of said layered crystal structure on a substrate by Molecular Beam Epitaxy (MBE) method with introducing oxygen-containing gas which is excited by irradiation of microwave, improvement in that a film-forming operation by the MBE method is interrupted temporarily after predetermined numbers of constituent layers which correspond to one unit crystal or less than one unit crystal are layered so that the deposited constituent layers are left in an activated oxygen atmosphere to effect a crystallization promotive operation, before the next film-forming operation is restarted.

This is a division of application Ser. No. 07/604,896 filed Oct. 31, 1990, now U.S. Pat. No. 5,143,896 issued Sept. 1, 1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing a superconducting thin film of oxide, more particularly, to an improved process for depositing a thin film of superconducting compound oxide on a substrate by Molecular Beam Epitaxy (hereinafter, MBE) technique.

The present invention relates also to a system for carrying out the process automatically by a computer.

2. Description of the Related Art

Thin films of compounds are prepared by so-called reaction-evaporation method in which materials are evaporated in an active gas atmosphere so that vapors of the materials are reacted with the active gas on a substrate to deposit a thin film of the compound on the substrate. When the compound is an oxide, oxygen is usually used as the active gas. In Tatsumi et al. "Epitaxial film growth of artificial (Bi--O)/(Sr--Ca--Cu--O) layered structure", Appl. Phys. Lett. 54 (23), Jun. 5, 1989, p. 2364-2366, an oxide superconducting thin film of Bi--Sr--Ca--Cu--O system is grown epitaxially by ion-beam sputtering technique in the presence of oxygen gas. It is also proposed to use excited oxygen which is activated by microwave in order to obtain an oxide thin film of high quality.

Thin films of compounds composed of more than two elements am advantageously prepared by the MBE technique in which proportions of constituent elements of the compound can be controlled precisely. A combination of the MBE and the reaction-evaporation is much effective for some compounds. D. G. Schlom et al. "Molecular beam epitaxy of layered Dy--Ba--Cu--O compounds", Appl. Phys. Lett. 53 (17), Oct. 24, 1988, p. 1660-1662, disclose a modified MBE process in which a thin film of oxide superconductor of Dy--Ba--Cu--O is prepared with introducing excited oxygen gas which is activated by microwave.

In a high-temperature superconductor possessing a layered perovskite crystalline structure composed of constituent layers each containing different elements, the constituent layers are stratified successively on a substrate. Superconductive property of a final product is influenced critically by the content of oxygen atoms in constituent layers. Therefore, it is extremely important to control supply of oxygen gas to each constituent layer. In fact, it is difficult to obtain a desired crystal structure of high-temperature oxide superconductor by the known modified MBE process in which constituent layers are deposited continuously without controlling the oxygen supply, so that the quality of the final product obtained by the known modified MBE is rather poor.

An object of the present invention is to solve the problem and to provide an improved MBE process for preparing thin films of oxide superconductors of high quality and a system for carrying out the process automatically by a computer.

SUMMARY OF THE INVENTION

The present invention provides a process for preparing a thin film of oxide superconductor having a layered crystal structure by depositing constituent layers of said layered crystal structure on a substrate by Molecular Beam Epitaxy (MBE) method while an oxygen-containing gas which is excited by irradiation of microwave is introduced in the neighborhood of a surface of the substrate, characterized in that, after predetermined numbers of constituent layers which correspond to one unit crystal or less than one unit crystal are layered, a film-forming operation by the MBE method is interrupted temporarily and the deposited constituent layers are maintained in an activated oxygen atmosphere in order to effect a crystallization promotive operation and then next film-forming operation by the MBE method is restarted after completion of said crystallization promotive operation, a combination of the film-forming operation and the crystallization promotion operation being repeated for desired times until a desired film thickness is obtained.

In the process according to the present invention, relative displacement of atoms in the crystal is promoted or accelerated in a stage of the crystallization promotion operation and satisfactory amount of oxygen is supplied to deposited layers, so that freshly deposited constituent layers are rearranged to a desired crystal structure which is indispensable to realize a perfect superconductor. The thin film of the final product obtained by the process according to the present invention shows a very smooth surface of high quality whose crystallinity is improved. This advantage is difficulty obtained by the known process in which constituent layers are deposited continuously without interposing the crystallization promotive operation according to the present invention.

Numbers of constituent layers after deposition of which the film-forming operation by the MBE method should be interrupted temporarily depend on kinds of oxides to be deposited. Generally, the crystallization promotion operation can be effected after numbers of constituent layers which correspond to one unit crystal are layered. When the crystal structure of an oxide to be deposited has many constituent layers that are difficult to be controlled, the crystallization promotion operation may be effected after numbers of constituent layers which correspond to less than one unit crystal, for example a half of unit crystal, are layered.

The crystallization promotive operation can be carried out by maintaining the deposited constituent layers in an oxygen-containing atmosphere. Termination of this crystallization promotive operation can be detected by monitoring an output signal or a pattern of a film analyzer such as a reflection high energy electron diffraction (RHEED) analyzer which is used commonly in MBE machines. In fact, high-temperature oxide superconductors show specific modulated output patterns which am inherent to respective oxides in the RHEED analyzer, in other words, the quality of deposited thin films can be evaluated by the RHEED pattern. Therefore, the crystallization promotive operation can be terminated when a predetermined or desired output signal of the analyzer is observed. For example, in a case of Y₁ Ba₂ Cu₃ O_(7-z) whose crystal has a longer lattice constant along b-axis than along a-axis, the quality of its thin film can be evaluated by measuring spaces of streaks in the RHEED pattern. An oxide superconductor of Bi₄ Sr₄ Ca₄ Cu₆ O₁₆₊δ shows a unique modulated RHEED pattern which can be used to determine termination of the crystallization promotive operation.

The process according to the present invention is applicable to any oxide superconductor which has a layered crystal structure as follows:

(1) (La_(1-x) α_(x)) ₂ CuO_(4-y)

in which α stands for Sr or Ba, "x" and "y" are numbers each satisfying respective range of 0≦x<1; and 0≦y<1,

(2) La₁ Ba₂ Cu₃ O_(7-z)

in which "Ln" stands for at least one element selected from the group comprising La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb and Lu and "z" is a number satisfying a range of 0≦z<1, and

(3) Bi₄ (Sr_(1-k) Ca_(k))_(m) Cu_(n) O_(p+)δ

in which "k", "m", "n", "p" and "δ" are numbers each satisfying respective range of 0≦k<1, 6≦m≦10, 4≦n≦8, p=6+m+n and -2≦δ≦+2. In this oxide, following proportions are preferable:

0.4<k<0.6, 7≦m≦9, 5≦n≦7 and -2≦δ≦+2,

0.2<k<0.4, 6≦m≦7, 4≦n≦5 and -2≦δ≦+2,

0.5<k<0.7, 9≦m≦10, 7≦n≦8 and -2≦δ≦+2.

This oxide can be represented by the following general formula:

    Bi.sub.2 (Sr.sub.1-k Ca.sub.k).sub.m/2 Cu.sub.n/2 O.sub.(p+δ)/2

because one unit crystal of the oxide is composed of two structures of this formula which are stratified symmetrically.

(4) Bi_(2-q) Pb_(q) Sr₂ Ca_(r-1) Cu_(r) O.sub.γ

in which "q", "r" and "δ" are numbers each satisfying respective range of 0.1≦q≦1, 1≦r≦6 and 6≦γ≦16.

It is apparent that the process according to the present invention is applicable also to the other high-temperature oxide superconductors having a layered crystal structure such as Ti-containing oxide superconductors, no-copper containing oxide superconductors or the like.

The substrate used in the process according to the present invention has preferably the same or similar lattice constant as the oxide to be deposited and is preferably selected from single crystals of oxides such as MgO, SrTiO₃, YSZ, LaGaO₃, LaGaO₃ and LaAlO₃. In a special case, a single crystal of silicon may be used. If desired, a buffer layer is interposed between the substrate and the thin film of superconducting oxide in order to prevent mutual migration of elements and/or to compensate mismatch in lattice constant. The thin film of superconducting oxide is preferably deposited on a predetermined plane of the substrate. In the case of single crystal oxide substrate of MgO and SrTiO₃, a {100} plane or a {110} plane is preferably used. The substrate is preferably heated during the operations of the process according to the invention at a temperature between 550° and 750° C.

The oxygen-containing gas is preferably pure oxygen gas which is excited by irradiation of microwave. This excited oxygen gas is supplied from straight nozzle(s) or ring nozzle(s) which are positioned in the neighborhood of a surface of the substrate where the nozzle doesn't intersect the beams of evaporated elements. A flow rate of excited oxygen gas depends on kinds of oxides to be deposited and/or capacity of evacuation pumps and can be determined by experiments. The flow rate of excited oxygen gas is usually selected between 0.1 and 50 SCCM.

The process according to the present invention can be carried out in a known modified MBE machine comprising a chamber in which a high vacuum can be created, means for introducing oxygen gas into the chamber, a microwave generator for exciting the oxygen gas, means for heating the substrate positioned in the chamber, means for controlling independently temperatures of evaporation sources, means for controlling independently shutters for evaporation sources, and, an analyzer for monitoring crystal structures of thin films deposited on the substrate such as RHEED.

The process according to the present invention is preferably automated by using a computer, preferably a micro processor, connected to the known MBE machine. Therefore, the present invention provides a system for depositing a thin film of oxide superconductor having a layered crystal structure on a substrate by the above-mentioned molecular beam epitaxy (MBE) machine.

The system according to the present invention is characterized in that the crystallization promotive operation is controlled by a computer in such a manner that, during the crystallization promotive operation, output image signals from the analyzer for monitoring crystal structures of a thin film deposited on the substrate is compared with image data of a known superconducting oxide stored in a memory of the computer and that next film-forming operation is restarted after the output image signals from the analyzer become substantially identical with the image data stored in the memory.

The analyzer can be a reflection high-energy electron diffraction (RHEED) analyzer and the image data of known superconducting oxide stored in the memory of computer which is utilized in the crystallization promotive operation can be a modulated pattern of the RHEED analyzer which is inherent to the superconducting oxide. In practice, the output image signals from the RHEED analyzer are converted to digital image data in an image processor and the digital image data are compared with image data of known crystal structures of a superconducting oxide stored in a memory in the computer.

It is also possible to modify operational factors such as partial pressure of oxygen in the chamber and substrate temperature, if necessary, during the crystallization promotive operation.

In a preferred system according to the present invention, both of the film-forming operation as well as the crystallization promotive operation are automated by the computer. In this case, all operational, factors including partial pressure of oxygen in the chamber, power of the microwave generator, substrate temperature, temperatures and shuttering order of evaporation sources during the film-forming operation are input previously in the computer and the following sequence is programmed in the computer:

(1) a film-forming operation in the MBE machine is interrupted temporarily after predetermined numbers of constituent layers which correspond to one unit crystal or less than one unit crystal are layered,

(2) output image signals from the analyzer is compared with image data of a known superconducting oxide stored in a memory of the computer, so that deposited constituent layers are left in an activated oxygen atmosphere,

(3) next film-forming operation is not restarted until the output image signals from the analyzer become substantially identical with the image data stored in the memory, and

(4) a combination of the film-forming operation and the crystallization promotion operation is repeated for desired times until a desired film thickness is obtained.

Now, the present invention is explained with reference to FIG. 1.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a simplified MBE machine which is used for producing the compound oxide thin film according to the present invention.

FIG. 1 shows essential parts of a molecular beam epitaxy (MBE) machine which is used in the present invention. All details of a pressure sensor in the chamber, manipulators, evacuation pumps or the like are omitted for clarity and because they are well-known. The MBE machine shown in FIG. 1 has a chamber (1) which can be evacuated to a high vacuum, a plurality of evaporation cells (2) such as Knudsen Cells each containing an evaporation source (10) or an element to be evaporated which are heated adjustably to desired temperatures, shutters (8) each being actuated independently and in a predetermined order so that the amount of each atom beam evaporated from the evaporation cell (2) is controlled, a substrate holder (3) equipped with a heater (4) whose temperature is controllable, a pipe (6) for introducing active gas such as oxygen gas which is opened in the neighborhood of a surface of a substrate (5) on which a thin film is deposited, a microwave generator (7) for exciting the active gas in the pipe (6), and an analyzer for monitoring a crystal structure of the thin film on said substrate such as RHEED analyzer consisting of an electron gun (20) and a screen (21).

The process according to the present invention can be carried out by operation the MBE machine manually. Following are two examples of manual operation when thin films of superconducting oxides: Y₁ Ba₂ Cu₃ O_(7-z) and Bi₄ Sr₄ Ca₄ Cu₆ O₁₆₊δ(= 2 Bi₂ Sr₂ Ca₂ Cu₃ O₈₊δ/2) are prepared.

A layered superconducting oxide of Y₁ Ba₂ Cu₃ O_(7-z) has a unit crystal which consists of a Ba--O layer, a Cu--O layer, a Ba--O layer, a Cu--O layer, a Y--O layer, a Cu--O layer, a Ba--O layer, a Cu--O layer and a Ba--O layer stratified in this order. When a thin film of Y₁ Ba₂ Cu₃ O_(7-z) is deposited on a substrate by manual operation, beams of vapour sources (10) each being an elemental metal of Y, Ba and Cu heated in a respective K-cell (2) are directed to a substrate (5) in a predetermined order by shuttering respective shutters (8), while excited oxygen gas is supplied to a surface of the substrate from the pipe (6), so that the above-mentioned Ba--O layer, Cu--O layer, Ba--O layer, Cu--O layer, Y--O layer, Cu--O layer, Ba--O layer, Cu--O layer and Ba--O layer are deposited successively in this order. This stage is called "film-forming operation". According to the present invention, after lamination of one unit crystal is complete, the film-forming operation is interrupted and a crystallization promotive operation is effected. In actual operation, all shutters (8) are closed and the constituent layers deposited are left in an atmosphere of oxygen gas which is supplied continuously through the pipe (6). During this crystallization promotive operation, rearrangement of elements or ordering of the crystal is realized. Timing of termination of the crystallization promotive operation can be detected by monitoring output signal of the reflection high energy electron diffraction (RHEED) analyzer. In the case of superconducting oxide of Y₁ Ba₂ Cu₃ O_(7-z), the crystallization promotive operation is terminated when a unique streak pattern caused by such a fact that the crystal has a longer lattice constant along b-axis than along a-axis is observed in the RHEED pattern. After the crystallization promotive operation complete, the next film-forming operation is restarted. A combination of the film-forming operation and the crystallization promotion operation is repeated for desired times until a desired film thickness is obtained.

A layered superconducting oxide of Bi₄ Sr₄ Ca₄ Cu₆ O₁₆₊δ (=2Bi₂ Sr₂ Ca₂ Cu₃ O₈₊δ/2) has a half (1/2) unit crystal which consists of a Bi--O layer, a Sr--O layer, a Cu--O layer, a Ca--O layer, a Cu--O layer, a Ca--O layer, a Cu--O layer, a Sr--O layer and a Bi--O layer stratified in this order. One unit crystals is made by two of the half unit crystal which are deposited symmetrically. When a thin film of Bi₄ Sr₄ Ca₄ Cu₆ O₁₆₊δ is deposited on a substrate by manual operation, it is preferable to carry out the crystallization promotion operation after half unit crystal is layered in the film-forming operation because numbers of constituent layers corresponding to one unit crystal are rather too larger to facilitate the ordering of crystal so that it is difficult to obtain a desired crystal structure composed mainly of a high-temperature phase of this superconducting oxide. In actual operation, beams of vapour sources (10) each being an elemental metal of Bi, Sr, Ca and Cu heated in a respective K-cell (2) (in this case the numbers of K-cells are increased to four) are directed to a substrate (5) in a predetermined order by shuttering respective shutters (8), while excited oxygen gas is supplied to a surface of the substrate from the pipe (6), so that the above-mentioned constituent layers of Bi--O layer, Sr--O layer, Cu--O layer, Ca--O layer, Cu--O layer, Ca--O layer, Cu--O layer, Sr--O layer and Bi--O layer which correspond to the half unit crystal are deposited successively in this order. According to the present invention, after lamination of half unit crystal is complete, the film-forming operation is interrupted and a crystallization promotive operation is carried out by closing all shutters (8) and deposited constituent layers are left in an atmosphere of oxygen gas which is supplied continuously through the pipe (6). During this crystallization promotive operation, rearrangement of elements or ordering of the crystal is realized. Timing of termination of the crystallization promotive operation can be detected by monitoring output signal of the RHEED analyzer. In the case of superconducting oxide of Bi₂ Sr₂ Ca₂ Cu₃ O₈₊δ2, the crystallization promotive operation is terminated when a specific modulation pattern which is unique to the high-temperature phase of this oxide is observed in the RHEED pattern. After the crystallization promotive operation is complete, the next film-forming operation is restarted. A combination of the film-forming operation and the crystallization promotion operation is repeated for desired times until a desired film thickness is obtained.

When the process according to the present invention is automated by the computer-aided system according to the present invention, the crystallization promotion operation is controlled by the computer (11) which is programmed as follows:

(1) a pattern which is created by the electron gun (20) on a screen (21) is transformed into electric image signals by an image processor (not shown). The electric image signals are input in the computer (11) and are compared with known image data of an oxide concerned which are stored in the computer,

(2) the deposited constituent layers are left in an activated oxygen atmosphere in order to effect a crystallization promotive operation until the electric image signals from the image processor become equal to the known image data,

(3) after the electric image signals from the image processor become equal to the known image data, the next film-forming operation is restarted, and

(4) a combination of the film-forming operation and the crystallization promotion operation is repeated for desired times until a desired film thickness is obtained.

In a preferred embodiment of the system according to the invention, all sequence during the film-forming operation and the crystallization operation are proceeded according to a program input in the computer (11). In this case, all operational parameters including a flow rate of oxygen gas (6), an energy of the microwave generator (7), temperature of the substrate heater (4), temperature of K-cells (2) and order and time duration of shutters (8) in the film-forming operation are memorized in the computer previously, so that all operations in the film-forming stage are carried out automatically under a control of the computer according to a programmed feed-back control sequence which is well-known in the computer control system.

According to the present invention, thin films of oxide superconductors of high quality can be obtained owing to the crystallization promotion operation which is interposed between successive film-forming operations.

The process according to the present invention can be carried out in known MBE machine without changing the machine and is advantageously operated automated condition by using a computer.

Now, examples of the present invention will be shown but the present invention should not be limited to the special examples.

EXAMPLE 1

A thin film of YBa₂ Cu₃ O_(7-z) was deposited on a {100} plane of a single crystal of MgO according to the process of the present invention in the MBE machine shown in FIG. 1. In this example, the computer (11) was not used but the operation was carried out in manual control.

Oxygen gas was excited by the microwave generator (7) and was supplied from the pipe (6) in the neighborhood of a surface of the MgO substrate (5). K-cells (2) each containing elemental metals of Y, Ba and Cu were heated to temperatures of 1,450° C. for Y, 500° C. for Ba and 1,080° C. for Cu. After then, a film-forming operation was started.

In the film-forming operation, after the chamber (1) was evacuated to a pressure 5×10⁻⁶ Torr and the substrate (5) was heated to a temperature of 650° C., shutters (8) were opened in an order of Ba, Cu, Ba, Cu, Y, Cu, Ba, Cu and Ba to deposit constituent layers corresponding to one unit crystal. Shutters were opened for 4 sec in Y, for 3 sec in Ba and for 5 sec in Cu respectively.

Then, a crystallization promotive operation was carried out by monitoring the RHEED analyzer. During the crystallization promotive operation, the pressure in the chamber and the substrate temperature was maintained at the same values as those of the film-forming operation, namely 5×10⁻⁶ Torr and 650° C. until an a-axis became shorter than b-axis in a RHEED pattern.

After this condition was realized, the film-forming operation was restarted. Then, a combination of the film-forming operation and the crystallization promotive operation was repeated until a desired film thickness of 100 nm was obtained.

For comparison, the film-forming operation alone was repeated without interposing the crystallization promotive operation of the present invention to prepare a thin film of 100 nm thick according to a known process. Film forming speed of the comparative example was 0.5Å/sec.

Common conditions to the present invention and the comparative example are as follows:

    ______________________________________                                         Substrate temperature: 650° C.                                          Pressure in the chamber:                                                                              5 × 10.sup.-6 Torr                                Flow rate of activated oxygen:                                                                        0.5 SCCM                                                Pressure in the oxygen supply pipe                                                                    0.5 Torr                                                at the microwave discharge zone:                                               ______________________________________                                    

Superconducting properties (Tc=critical temperature, Jc=critical current density) of thin films obtained are summarized in Table 1.

                  TABLE 1                                                          ______________________________________                                                Power of  Tc      Jc*      Appearance                                          microwave (W)                                                                            (K.)    (A/cm.sup.2)                                                                            of surface                                   ______________________________________                                         Invention                                                                               150         88      2.4 × 10.sup.6                                                                  smooth                                     Comparative                                                                             150         82      0.9 × 10.sup.4                                                                  uneven                                     ______________________________________                                          (*): determined at 77° K.                                         

The result revealed that the thin film prepared by the process according to the present invention is improved in surface condition and in crystallinity and hence is of high quality.

EXAMPLE 2

Example 1 was repeated to prepare a thin film of YBa₂ Cu₃ O_(7-z) on a {100} plane of a single crystal of MgO in the MBE machine shown in FIG. 1, but, in this example, all operations were programmed in the computer (11) in which the following data were input previously:

    ______________________________________                                         Film-forming operation                                                         Temperature of K-cells:                                                                           Y = 1,450° C., Ba = 500° C.                                      Cu = 1,080° C.                                       Shuttering order:  Ba, Cu, Ba, Cu, Y, Cu, Ba,                                                     Cu and Ba                                                   Shuttering time duration:                                                                         Y = 4 sec, Ba = 3 sec,                                                         Cu = 5 sec.                                                 Pressure in the chamber:                                                                          5 × 10.sup.-6 Torr                                    Substrate temperature:                                                                            650° C.                                              Flow rate of activated oxygen:                                                                    0.5 SCCM                                                    Pressure in the oxygen supply pipe                                                                0.5 Torr                                                    at the microwave discharge zone:                                               Film thickness:    100 nm                                                      Crystallization promotive operation                                            Pressure in the chamber:                                                                          5 × 10.sup.-6 Torr                                    Substrate temperature:                                                                            650° C.                                              Flow rate of activated oxygen:                                                                    0.5 SCCM                                                    Pressure in the oxygen supply pipe                                                                0.5 Torr                                                    at the microwave discharge zone:                                               RHEED pattern data:                                                                               A streak pattern of known                                                      crystal structure of                                                           YBa.sub.2 Cu.sub.3 O.sub.7-z was                                               converted to digital image                                                     data                                                        ______________________________________                                    

The film-forming operation was carried out automatically according to the program. In the crystallization promotive operation, the computer (11) was programmed such a manner that output signals of the RHEED analyzer were processed by an image processor to produce image data which were compared with the streak pattern data of known superconducting oxide of Y₁ Ba₂ Cu₃ O_(7-z) stored in the computer (11) and that timing of termination of the Crystallization promotive operation was determined when both data became substantially identical, more precisely when distances of a-axis and distances of b-axis became identical.

The result of superconducting properties of thin film obtained is summarized in Table 2.

                  TABLE 2                                                          ______________________________________                                         Reaction Power of        Tc     Jc*                                            Gas      microwave (W)   (K.)   (A/cm.sup.2)                                   ______________________________________                                         Oxygen   150             88     2.4 × 10.sup.6                           ______________________________________                                          (*): determined at 77° K.                                         

The thin film obtained showed improved surface condition and crystallinity and hence was of high quality.

EXAMPLE 3

A thin film of Bi₂ Sr₂ Ca₂ Cu₃ O₈₊δ/2 was deposited on a {100} plane of a single crystal of MgO according to the process of the present invention in the same MBE machine as FIG. 1, but numbers of K-cells (2) were increased to four. In this example, the computer (11) was not used but the operation was carried out in manual control.

Oxygen gas was excited by the microwave generator (7) and was supplied from the pipe (6) in the neighborhood of a surface of the MgO substrate (5). K-cells (2) each containing elemental metals of Bi, St, Ca and Cu were heated to temperatures of 530° C. for Bi, 500° C. for Sr, 520° C. for Ca and 1,100° C. for Cu. After then, a film-forming operation was started.

In the film-forming operation, after the chamber (1) was evacuated to a pressure of 5×10⁻⁶ Torr and the substrate (5) was heated to a temperature of 650° C., shutters (8) were opened in an order of Bi, Sr, Cu, Ca, Cu, Ca, Cu, Sr and Bi to deposit constituent layers corresponding to a half unit crystal. Shutters were opened for 2 sec in Bi, for 3 sec in Sr, for 4 sec in Ca and for 3 sec in Cu respectively.

After constituent layers corresponding to a half unit crystal were layered, a crystallization promotive operation was carried out by monitoring the RHEED analyzer. During the crystallization promotive operation, the pressure in the chamber and the substrate temperature were maintained at the same values as those of the film-forming operation, namely 5×10⁻⁶ Torr and 650° C. until a specific modulated RHEED pattern which is unique to the crystal of Bi₂ Sr₂ Ca₂ Cu₃ O_(8+d/2) was observed on a monitor.

After this condition was realized, the next film-forming operation was restarted. Then, a combination of the film-forming operation and the crystallization promotive operation was repeated until a desired film thickness of 100 nm was obtained.

For comparison, the film-forming operation alone was repeated without interposing the crystallization promotive operation of the present invention to prepare a thin film of 100 nm thick according to a known process. Film forming speed of the comparative example was 0.5Å/sec.

Common conditions to the present invention and the comparative example were as follows:

    ______________________________________                                         Substrate temperature: 650° C.                                          Pressure in the chamber:                                                                              5 × 10.sup.-6 Torr                                Flow rate of activated oxygen:                                                                        0.5 SCCM                                                Pressure in the oxygen supply pipe                                                                    0.5 Torr                                                at the microwave discharge zone:                                               ______________________________________                                    

Superconducting properties (Tc, Jc) of thin films obtained are summarized in Table 3.

                  TABLE 3                                                          ______________________________________                                                Power of  Tc      Jc*      Appearance                                          microwave (W)                                                                            (K.)    (A/cm.sup.2)                                                                            of surface                                   ______________________________________                                         Invention                                                                               150         108     2.8 × 10.sup.6                                                                  smooth                                     Comparative                                                                             150          92     0.9 × 10.sup.4                                                                  uneven                                     ______________________________________                                          (*): determined at 77° K.                                         

The result revealed that the thin film prepared by the process according to the present invention is improved in surface condition and in crystallinity and hence is of high quality.

EXAMPLE 4

Example 3 was repeated to prepare a thin film of Bi₂ Sr₂ Ca₂ Cu₃ O_(8+d/2) on a {100} plane of a single crystal of MgO in the same MBE machine as FIG. 1, but the numbers of K-cells were increased to four and all operations were programmed in the computer (11) to which the following data were input previously:

    ______________________________________                                         Film-forming operation                                                         Temperature of K-cells:                                                                           Bi = 530° C., Sr = 500° C.                                       Ca = 520° C. Ca =                                                       1,100° C.                                            Shuttering order:  Bi, Sr, Cu, Ca, Cu, Ca, Cu,                                                    Sr and Bi                                                                      (= numbers of constituent                                                      layers corresponding to a                                                      half unit crystal)                                          Shuttering time duration:                                                                         Bi = 2 sec, Sr = 3 sec,                                                        Ca = 4 sec, Cu = 3 sec.                                     Pressure in the chamber:                                                                          5 × 10.sup.-6 Torr                                    Substrate temperature:                                                                            650° C.                                              Flow rate of activated oxygen:                                                                    0.5 SCCM                                                    Pressure in the oxygen supply pipe                                                                0.5 Torr                                                    at the microwave discharge zone:                                               Film thickness:    100 nm                                                      Crystallization promotive operation                                            Pressure in the chamber:                                                                          5 × 10.sup.-6 Torr                                    Substrate temperature:                                                                            650° C.                                              Flow rate of activated oxygen:                                                                    0.5 SCCM                                                    Pressure in the oxygen supply pipe                                                                0.5 Torr                                                    at the microwave discharge zone:                                               RHEED pattern data:                                                                               A streak pattern of known                                                      crystal structure of                                                           Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+d/2                                was converted to digital                                                       image data                                                  ______________________________________                                    

The film-forming operation was carried out automatically according to the program. In the crystallization promotive operation, the computer (11) was programmed such a sequence that output signals of the RHEED analyzer were processed in an image processor to produce image data which were then compared with the streak pattern data of known superconducting oxide of Bi₂ Sr₂ Ca₂ Cu₃ O_(8+d/2) stored in the computer (11) and that the crystallization promotive operation was terminated when both data became identical, more precisely when a specific modulated pattern which is unique to the crystal of Bi₂ Sr₂ Ca₂ Cu₃ O_(8+d/2) was obtained in the RHEED.

The result of superconducting properties of thin film obtained is summarized in Table 4.

                  TABLE 4                                                          ______________________________________                                         Reaction Power of        Tc     Jc*                                            Gas      microwave (W)   (K.)   (A/cm.sup.2)                                   ______________________________________                                         Oxygen   150             108    2.8 × 10.sup.6                           ______________________________________                                          (*): determined at 77° K.                                         

The thin film obtained showed improved surface condition and crystallinity and hence was of high quality. 

What is claimed is:
 1. A process for preparing a film of oxide superconductor having a layered crystal structure by depositing constituent layers of said layered crystal structure on a substrate successively by molecular beam epitaxy (MBE) method while an oxygen-containing gas which is excited by irradiation of microwave is introduced in the neighborhood of a surface of the substrate, comprising the steps of depositing preselected numbers of constituent layers which correspond to one unit crystal or less than one unit crystal, interrupting the film-forming operation by the MBE method temporarily, maintaining the deposited constituent layers in an activated oxygen atmosphere in order to effect a crystallization promotive operation and then restarting a next film-forming operation by the MBE method after completion of said crystallization promotive operation which is terminated when a desired pattern is observed in a reflection high energy electron diffraction (RHEED) analyzer, a combination of said film-forming operation and said crystallization promotive operation being repeated for desired times until a desired film thickness is obtained.
 2. The method of claim 1 wherein said film-forming operation by the MBE method is interrupted temporarily after numbers of constituent layers which correspond to one unit crystal are layered.
 3. The method of claim 1 wherein said film-forming operation by the MBE method is interrupted temporarily after numbers of constituent layers which correspond to a half of unit crystal are layered.
 4. The process set forth in claim 1 wherein said desired pattern in the RHEED analyzer is a modulated pattern which is inherent to an oxide superconductor to be obtained.
 5. The process set forth in claim 1 wherein said desired pattern in the RHEED analyzer is a preselected streak pattern which is inherent to an oxide superconductor to be obtained.
 6. The process set forth in claim 1 wherein said oxide superconductor is an oxide selected from the group consisting of(1) (La _(1-x) α _(x)) ₂ CuO _(4-y) in which α stands for Sr or Ba, "x" and "y" are numbers each satisfying respective range of 0≦x<1 and 0≦y<1, (2) Ln, Ba₂ Cu₃ O _(7-z) in which "Ln" stands for at least one element selected from the group comprising La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb and Lu and "z" is a number satisfying a range of 0≦z<1, and (3) Bi ₄ (Sr _(1-k) Ca _(k)) _(m) Cu _(n) O _(p+)δ in which "k", "m", "n", "p" and "δ" are numbers each satisfying respective range of 0<k<1, 6≦m≦10, 4≦n≦8, p=6+m+n and -2≦δ≦+2. (4) Bi _(2-q) Pb _(q) Sr ₂ Ca _(r-1) Cu _(r) O .sub.γ in which "q", "r" and "δ" are numbers each satisfying respective range of 0.1≦q≦1, 1≦r≦6 and 6≦γ≦16.
 7. The process set forth in claim 1 wherein said substrate is selected from the group consisting of MgO, SrTiO₃, YSZ, LaGaO₃, LaGaO₃ and LaAlO₃.
 8. The process set forth in claim 1 wherein said substrate is heated at 550° to 750° C. during said film-forming operation and said crystallization promotive operation.
 9. The process set forth in claim 1, when an oxide superconductor of Y ₁ Ba ₂ Cu ₃ O _(7-z), in which "z" is a number satisfying a range of 0≦z<1, is prepared, said film-forming operation by the MBE method is interrupted temporarily after numbers of constituent layers which correspond to one unit crystal are layered.
 10. The process set forth in claim 1, when an oxide superconductor of Bi ₄ Sr ₄ Ca ₄ Cu ₆ O ₁₆₊δ, in which "δ" is a number satisfying a range of -2≦δ≦+2, is prepared, said film-forming operation by the MBE method is interrupted temporarily after numbers of constituent layers which correspond to a half unit crystal are layered. 